This invention relates to hollow containers with inert and/or impermeable inner surfaces and, more particularly, to such containers produced by the application of a thin coating either directly to the container preform or to the core rods of the preform injection machine.
Plastic containers have been replacing glass in many applications where easy handling, low weight and non-breakability are needed. To date, polymers have had varying degrees of inertness to the packaged content which differ from the inertness of glass. In the case of plastic food packages, surface inertness helps diminish potential desorption of packaging material components into the food, to prevent flavor absorption, to avoid loss of food constituents through the package walls and to avoid ingress of air or other substances from outside the package.
Refillable plastic packages add a further dimension to inertness requirements because these packages must withstand washing and refilling. Such containers should not absorb contact materials including, inter alia, washing agents and materials stored in the container.
Packages for carbonated beverages are pressurized and must withstand considerable mechanical stress in handling. It is therefore difficult for a single material to provide the necessary mechanical stability and the required inertness.
Current plastic packages for carbonated beverages consist of either a single material, e.g. polyethylene terephthalate (PET), or of multi-layer structures where the middle layers normally provide the barrier properties while the outer layers provide the required mechanical strength.
Multi-layer containers are produced either by coinjection or coextrusion but these processes restrict choice of materials and cannot provide a very thin inner layer.
Therefore, a plastic container with an impermeable, dense, "glass-like" inner surface cannot be produced by conventional methods because these limit the options for the internal surface. Where a plastic, such as high-crystalline PET has good barrier properties but poor transparency, a very thin inner layer is needed so that the transparency of the container as a whole is not impaired. Where a plastic, intended as inner layer, has a different glass transition temperature than the main container material, it cannot be blow-molded unless the inner layer is very thin and can be subjected to individual heat treatment. And, where a barrier plastic has residual monomers or depolymerization by-products, such as acetaldehyde for PET, these can be extracted or deaerated from a very thin layer but not from a thick layer. Accordingly, more polymer options are possible with very thin layer structures.
Recycling is yet another dimension insofar as mass-produced packages are concerned. The reuse of recycled plastic for same purpose, that is to produce new containers ("closed-loop" recycling) is an issue which has attracted much attention, and for PET, this has been achieved to-date by depolymerizing the recycled material in order to free it of all trace contaminants which might otherwise migrate and come in contact with the container content. An impermeable inner layer, which is the purpose of this invention, would enable recycled material to be reused directly for new containers i.e. without special treatment such as depolymerization, since traces of foreign substances could no longer contact the container's content. This would simplify the "closed-loop" recycling process considerably by obviating the need for depolymerization.
Furthermore, recyclability within established recycling systems, both "open-loop", i.e. recycling for other uses, or "closed-loop" i.e. reuse for same purpose, is necessary for any mass-produced package. In "open-loop" systems, the normal method is to separate, clean and chop up the plastic into small flakes. The flake is then either melted and used for molding other objects, or for fibre production. For this form of recycling, it is important to note that any contaminant to the main plastic, such as a coating, should effectively be present in negligible quantities and, preferably, be solid and insoluble within the molten plastic so that it can be filtered off prior to sensitive applications, such as fibre-production. PET is also recycled in "closed-loop" by depolymerization and it is important that the coating material should be unchanged by this process, be insoluble in the monomers resulting from the process, and be easily separable from these monomers. With a correct choice of material, a thin, inner layer can fulfill these criteria.
Finally, since one option available when using a single very thin layer is to use a highly crystalline version of the same polymer as is used in the main part of the container, e.g. highly crystalline PET in PET containers, the inner layer is virtually the same as the outer and recycling presents no problems.
As a result of these inherent limitations, current technology cannot now produce containers with a high barrier inert inner layer having a good appearance because it forces compromises which detract from the desired end result which is an improved beverage container.